A flock of construction cranes fills the sky. Downtown's population is soaring. The skyline is changing and so is the texture of city life.

A generation ago, West Madison Street was Chicago's Skid Row home to winos and flophouses. It is now a chic strip of sushi joints, cycle studios, preschools and a gourmet ice cream shop. But the architectural fare consists of bland apartment high rises that have drawn complaints of monotony.

In River North, the old Ed Debevic's, a faux '50s diner best known for gum-snapping, table-dancing waiters and waitresses, is gone, replaced by an eye-grabbing apartment tower whose cantilevered wedges of glass resemble a Jenga game in midstream. Yet that building is an anomaly amid the bland, form-follows-finance high-rises popping up elsewhere in the neighborhood and around the city.

So it goes in The Great Chicago Post-Recession Building Boom. A surge of tall buildings, the vast majority of them housing rental apartments, is creating a densely populated, urban core — call it the Super Loop — that is pushing far beyond the borders of the traditional downtown. But the Super Loop is patently un-super in at least one respect: It lacks a new version of the technological and aesthetic innovations that made Chicago's reputation as the cradle of modern architecture.

As Mayor Rahm Emanuel prepares to host the second edition of a global architecture biennial that will exhibit cutting-edge design ideas, most of the new high-rises are based on tired commercial formulas. They are merely better versions of the exposed-concrete boxes, stacked atop parking garages, that have marred the blocks west of North Michigan Avenue. They make the leap, in other words, from awful to mediocre.

To be sure, there are exceptions, like the under-construction Vista Tower, by Chicago architect Jeanne Gang, a staggered mountain of glass that will become the city's third tallest building when it is completed in 2020. But the quality of a city's built environment is determined far more by the typical building than the exceptional one. And there are still too many typical buildings that are banal, graceless expressions of architect Cass Gilbert's immortal observation: A skyscraper "is a machine that makes the land pay."

Consider, though, the bright side of the building boom, which has been overshadowed by the violence wracking parts of Chicago's West and South sides. For city lovers who believe that density, rather than sprawl, is the ideal path to cutting car use, saving energy and halting the effects of climate change, these are, in many ways, the best of times:

—Nearly 229,000 people now live in the central area roughly bounded by Lake Michigan on the east, the Stevenson Expressway on the south, Ashland Avenue on the west and North Avenue on the north, according to an analysis of population data done for the Tribune by Chicago-based demographer Rob Paral.

That's an increase, since 1990, of more than 82,000 — more than the population of Evanston. Three-quarters of the gain has occurred since 2000 as waves of high-rise residential construction — first primarily condominiums, then, after the recession, chiefly rental apartments — remade the city's core.

Taken by itself, the Super Loop would form Illinois' second-largest city, easily topping No. 2 Aurora, which has about 201,000 residents.

—The population surge is mirrored (literally) in the skyline and its myriad glass-sheathed towers. Since 2010, developers here have erected more than 70 high-rises and started construction on at least another 40 citywide — most in the Super Loop, according to figures compiled for the Tribune by the Chicago-based Council on Tall Buildings and Urban Habitat, which tracks skyscraper construction worldwide.

Because those figures are limited to structures that are at least 12 stories tall, they actually understate the building boom's impact. On the former site of Oprah Winfrey's Harpo Studios in the West Loop, for example, developer Sterling Bay is erecting a nine-story headquarters for McDonald's Corp. It soars above the old meatpacking houses that give the area its grit and exemplifies, by virtue of its location west of the Kennedy Expressway, how downtown has spread far beyond its longtime borders.

The three most active high-rise construction zones are River North, the West Loop and the South Loop. The Loop, birthplace of the skyscraper and still the place where tourists flock for docent-led tours to explore its development, ranks fourth.

Few would have predicted this burst nine years ago, when the Great Recession froze the city's skyline in place and left an embarrassing hole in the ground, still unfilled, on the construction site of the Santiago Calatrava-designed Chicago Spire, which was to be a 2,000-foot-tall, twisting condominium skyscraper.

As the economy has strengthened, developers have moved to capitalize on the demand for city living, particularly among members of the millennial generation. Telltale signs of their presence: stylish, 20-somethings walking tiny dogs, pushing baby strollers and getting around on Divvy bikes.

"It's the re-urbanization of America," said John Lahey, chairman of Solomon Cordwell Buenz, a Chicago-based firm that specializes in residential high-rises.

It's also a shift in the urban map: The once-frayed edges of downtown, home to the poor and working-class, are now the glittering home of the affluent. Rental rates, while less expensive than on the coasts, still leave many priced out. City officials last month proposed a pilot program to generate affordable housing in gentrifying areas of the Near North and Near West sides as well as along Milwaukee Avenue. But changing the trajectory of the marketplace won't be easy.

It is also proving difficult to live up to the legacy of Chicago's past building booms, which in the 1880s produced the first skyscrapers and in the 1960s and 1970s pioneered structural advances that allowed skyscrapers like the 108-story Willis Tower to rise to unprecedented heights. While the city's reputation as a wellspring of innovation derives chiefly from its tall office buildings, its residential towers, such as Ludwig Mies van der Rohe's structurally expressive high-rises at 860 and 880 North Lake Shore Drive, have been equally influential.

Very little in the pre-Recession building boom lived up to that high standard. West of North Michigan Avenue, in River North, it produced so many exposed-concrete eyesores — boxy residential high-rises stacked above blank-walled parking garages—that former Mayor Richard M. Daley was forced to pledge in 2002 that there would be "no more ugly buildings."

The new wave of towers isn't ugly, but is it inspiring and innovative? Hardly.

To assess its impact, head to North Wells Street in River North, where three apartment high-rises have transformed an outpost of the fading car culture.

A Howard Johnson motel bit the dust to make way for the 34-story Exhibit on Superior. A glitzy Planet Hollywood (later a Gino's East), originally adorned with fake metal versions of pink and green searchlights, has been replaced by The Gallery on Wells, a 39-story high-rise. And the theme park-y Ed Debevic's came down for the 23-story 640 North Wells, a glassy eye-grabber.

In contrast to the car-is-king approach of their predecessors, some of these buildings introduce touches of pedestrian-oriented urbanity.

Exhibit on Superior, for example, is set back from Wells, making room for a patch of greenery and restaurant seating that provides much-needed open space. Apartments wrap the perimeter of the tower's parking garage podium, preventing the dreaded blank-walled look. At 640 North Wells, the parking garage is clad in a handsome veneer of porcelain. City officials have encouraged such changes with new regulations and design guidelines.

"I think there have been quite a few lessons learned" from the pre-Recession building boom, said David Reifman, commissioner of the city's Department of Planning and Development.

Yet you can't legislate good design. The Gallery on Wells, by Loewenberg Architects, is essentially a reprise of the firm's hulking towers set atop parking garages. It's just dressed-up in brick in a weak attempt to relate to the low-rise brick buildings of the nearby gallery district from which it takes its name. Exhibit on Superior, by bKL Architecture, is an ordinary box, albeit with more sophisticated brick tailoring. 640 North Wells, by Hartshorne Plunkard Architecture, at least breaks out of the box with its bold sculptural look.

Alex Milanoski, development chair of the River North Residents Association, a citzens group that monitors new construction, hopes the increased density brought by the new towers will attract new businesses that fill empty storefronts in the area. But the new high-rises, in his view, have made this stretch of Wells "pretty cavernous."

"I appreciate the higher density," Milanoski said, "but it definitely needs to be done in a mindful manner."

City officials are trying to strike such a balance in the booming West Loop, which attracts scores of visitors to its Randolph Street restaurant row and Fulton Market district yet is struggling to retain the character that made those areas attractive in the first place. Between 2000 and 2016, the West Loop's population surged to 10,860 from 4,409; the number of housing units skyrocketed to 6,408 from 2,111; and the number of households with children grew to 613 from 234.

Those figures are contained in a draft of West Loop urban design guidelines that the Department of Planning and Development issued last month. The guidelines were prompted, the draft says, by public concerns over a loss of existing character and scale in the West Loop, the density and height of new buildings, the need for access to sunlight, a lack of open space and the monotonous architecture of new developments. In response, the guidelines call for such things as avoiding abrupt changes in height, and building taller and thinner towers to allow more sunlight to penetrate to street level.

Such an approach can make a significant difference, as illustrated by 1001 S. State St., a 40-story South Loop apartment high-rise by Solomon Cordwell Buenz.

In contrast to a hulking, brick-faced tower across the street, the building's exterior is broken into guitar-like curves, faced in glass and canted at an angle to provide residents views of Willis Tower. Though 1001 South State's base contains a parking garage, it is sliced on a diagonal, opening up a ground-level seating area that brings a shot of street life to a section of the South Loop that lacks the vitality of River North and the West Loop.

Like 640 North Wells, 1001 South State tweaks the tower-on-a-parking-podium formula to good effect. But it remains a marginal shift, not a fundamental one. Elsewhere around the world, architects are exploring bold new paradigms.

In Milan, the wide balconies of a two-tower residential complex called the Vertical Forest contain hundreds of trees, as well as thousands of shrubs and plants. The design, by the Boeri Studio, is based on the concept of "biological architecture," which relies on a screen of vegetation, rather than conventional walls and mechanical systems, to filter sunlight, control temperatures and save energy.

In Singapore, a residential complex called Reflections at Keppel Bay links pairs of curving towers with landscaped sky decks that are assembled at ground level and jacked into place at different levels between the high-rises. The design by Studio Libeskind, integrates the decks into its multi-building silhouette — in contrast to the new Chicago high-rises, where recreation decks are typically plopped atop parking garage podiums.

While I haven't seen these buildings firsthand, it's clear that they open new doors with new thinking. Chicago architects are fully capable of such work, as their international projects show. But they often find their overseas clients more receptive to experimentation than the ones they work with here.

As the 2017 Chicago Architecture Biennial prepares to open, then, the city occupies the unaccustomed position of boasting more innovation in the horizontal realm — evident in the vibrant landscapes of Millennium Park, The 606 trail and the downtown Riverwalk — than in the vertical one. The growth of Chicago's Super Loop is a boon for the city. But the high-rise architecture, while an upgrade from the last building boom, remains underwhelming compared with the heights of technological and aesthetic innovation of the city's past.

The biennial, a showcase for new ideas and architectural talents, is coming at the right time.